For example EP 1 255 044 shows a three-position booster valve mechanism with a housing inside which a supply valve and an exhaust valve are biased into closed positions. The supply valve is placed inside a supply passage which extends between an inlet port and an outlet port. The exhaust valve is placed inside an exhaust passage which extends between an outlet port and an exhaust port. A stepped stem is provided for moving either the supply valve either the exhaust valve towards its open position. This stem extends slideable through guiding openings in the valves and in either one of two directions is able to come to rest with a stepped shoulder portion against one of the respective valves such that it can push that valve in that direction towards its open position. A piston-cylinder system is provided which at one side of its piston connects to the inlet port via an adjustable proportional regulator, and on the other side of its piston connects to the outlet port via a fluid passage. The adjustable proportional regulator is designed to output a pilot pressure in response to a control signal. The piston then is movable in response to a pressure differential between the inlet port and the outlet port. The piston is connected to the stem and thus is able to move the valves in one of three positions, that is to say one in which both valves are closed, one in which only the supply valve is opened, and one in which only the exhaust valve is opened.
A disadvantage with this is that the functioning of the booster valve mechanism leaves to be desired. It's construction is rather complex. Also a proper functioning of the booster valve mechanism cannot be guaranteed, for example because wear may occur between the stem and the guiding openings in the valves. This may lead to small abraded particles which are free to wander through the valve and for example may get stuck between sealing rings of the valves and complementary opposing seats for them. This then may lead to leakages starting to occur, particularly because the valves are pressure balanced and only have a spring urging them towards closed positions. The likelihood of such wear to start to occur between the stem and the valves is even enlarged because of the fact that the stem is directly or indirectly guided at a plurality of points. Not only is the stem guided slideable through the guiding openings in the supply and exhaust valves themselves, but also with a lower stem end within a separate distinctive guiding sleeve which is fixedly connected to the housing. Furthermore a sealing ring of the piston of the piston-cylinder system guides the stem slideable within the housing. Over a period of time this can cause a relative high friction for the piston, stem and valves to start moving. This high friction in return may lead to hysteresis and to a slower response time for the booster valve mechanism to react on a changing pilot pressure which is delivered in response to a control signal. This particularly is the case when the booster valve mechanism needs to be used under harsh conditions, like low temperatures which make the various sealing rings and guiding organs stiffer.
Another example of such a three-position booster valve mechanism is known from U.S. Pat. No. 8,205,632. Like the abovementioned booster valve mechanism, this known valve mechanism also has an inlet, outlet and exhaust port, and a piston-cylinder operated stepped stem which has shoulders for interaction with supply and exhaust valves to be moveable up and down. The piston-cylinder control system again is mounted on top of the housing at a position above the supply and exhaust valves. The stem is connected to the piston and extends downwardly therefrom through a guiding opening in the housing, through a guiding opening in the exhaust valve and through a guiding opening in the supply valve. Between each of those guiding openings and the stem, annular clearances are provided which are destined to form so-called fluid leak paths for operating air, so that pressure on both sides of the supply and exhaust valves is substantially balanced and so that a pressure present at the outlet port also gets exerted to a lower side of the piston. At its upper side the piston cylinder system is connected to a pilot port. The piston-cylinder system thus can move the stem and the valves in response to a difference between a pilot signal coming from the pilot port and the air pressure in the outlet port.
This booster valve mechanism as known from U.S. Pat. No. 8,205,632 substantially has the same disadvantages as the one of EP 1 255 044, that is to say its functioning leaves to be desired, and it's construction is rather complex, while a proper leakage-free functioning cannot be guaranteed. With this, wear between the valves and the stem is even more likely to occur because of the substantial amount of play between the stem and the guiding openings which are necessary for forming the fluid leak paths. This may lead to the stem starting to tilt or slant somewhat within one or more of the guiding openings, which may lead to a large degree of abrasion and to a relative high friction for the stem and valves to move relative to each other inside the housing.
The present invention aims to overcome the abovementioned disadvantages at least partly or to provide a usable alternative. In particular the invention aims to provide a safe and reliable three-position booster valve mechanism which is less susceptible to wear.